The invention relates to using a mechanical connection to assemble together parts made of thermostructural composite material, such as a ceramic matrix composite (CMC) material or a carbon/carbon (C/C) material, where such a material is typically constituted by a porous substrate, such as a porous fiber substrate, that is densified by a ceramic matrix. The fibers of the substrate may in particular be made of carbon or of ceramic. The matrix is made of a refractory ceramic such as, by way of example: a refractory carbide, nitride, boride, or oxide. Thermostructural composite materials are remarkable because of their mechanical properties that make them suitable for constituting structural elements, and because of their ability to conserve these properties at high temperatures.
The invention relates more particularly to the aerodynamic and thermal behavior of mechanical connections used for assembling together parts made of thermostructural composite material when the parts are to be immersed in a high-temperature stream, for example as when fabricating all or part of afterbody assemblies for aeroengines such as exhaust cones (also known as “exhaust plugs”), or flaps for a variable-section nozzle.
In these applications, the parts for assembling together are generally relatively fine (thickness of a few millimeters), and they need to satisfy aerodynamic requirements, which means that it is preferred to use rivets for making the mechanical connections between the parts. With assemblies used for making aeroengine afterbody parts, it is general practice to use rivets that present on one side of the assembly a flat head (i.e. a head of frustoconical shape with an end that is flat) that is received in a countersink formed in one of the parts to be assembled together, and on the other side, a column or a washer serving to take up differential expansion between rivets, which expand considerably, and the parts made of thermostructural composite material, which expand little. By incorporating the head of the rivet in a countersink present on one side of the assembly, it is possible to obtain good aerodynamic performance, but only on that side of the assembly, since the presence of the column and of a portion of the body of the rivet on the other side of the assembly gives rise to considerable drag.
Another solution, which is used in making flaps for a variable section nozzle, consists in connecting the parts made of thermostructural composite material together with rivets, such as those sold under the trademark Fybrfast®, which rivets have at one end a head and at the other end an end portion that is pressed and flattened in order to fasten the parts together. Nevertheless, with that type of rivet, the head and the flattened portion of the body of the rivet project from either side of the assembly and consequently they project beyond the streamlining of the assembly.